SP6120EY datasheet(11/22 Pages) SIPEX

11 / 22 page

Date: 1/21/05

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

Continuous

Load

Current

GH, GL

Voltage

Discontinuous

Load

Current

GH, GL

Voltage

TIME

Zero Crossing Detection

In some applications, it may be undesirable to have

negative conduction current in the inductor. This

situation happens when the ripple current in the

inductor is higher than the load current. Therefore,

the SP6120 provides an option for “discontinu-

ous” operation. If the Program Logic (see next

section) is set for discontinuous mode, then the

Driver Logic looks at the Zero Crossing Compara-

tor and the state of the lower gate driver. If the low

side MOSFET was “on” and V(ISP)-V(ISN) < 0

then the low side MOSFET is immediately turned

off and held off until the high side MOSFET is

turned “on”. When the high side MOSFET turns

“on” , the Driver Logic is reset. The following

figures show continuous and discontinuous opera-

tion for a converter with an NFET high side

MOSFET.

Discontinuous vs. Continuous Mode

The discontinuous mode is used when better light

load efficiency is desired, for example in portable

applications. Additionally, for power supply se-

quencing in some applications the DC-DC con-

verter output is pre-charged to a voltage through a

switch at start-up, and discontinuous operation

would be required to prevent reverse inductor

current from discharging the pre-charge voltage.

The continuous mode is preferable for lower noise

andEMIapplicationssincethediscontinuousmode

can cause ringing of the switch node voltage when

it turns both switches off. Another example where

continuous mode could be required is one where

the inductor has an extra winding used for an over-

winding regulator and thus continuous conduction

is necessary to produce this second output voltage.

Program Logic

The Program pin (PROG) of the SP6120 adds a

new level of flexibility to the design of DC/DC

converters. A 10

µA current flows either into or out

of the Program pin depending on the initial poten-

tial presented to the pin. If no resistor is present, the

Program Logic simply looks at the potential on the

pin, sets the mode to “continuous” and programs

NFET or PFET high side drive accordingly. If the

68k

Ω resistor is present,

Program Logic: continued

the voltage drop across the resistor signals the

SP6120 to put the Driver Logic in “discontinuous”

mode. With one pin and a 68k

Ω resistor, the

SP6120 can be configured for a variety of operat-

ing modes:

Ω

THEORY OF OPERATIONS: Continued

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